I have to add drainage to a yard, Currently trhe yard runs 80 feet from the back of the house to a ditch. Every spring and heavy rain, it becomes a swamp. This is bacuse the first 60 feet have a 2 inch drop in the right direction, but the last 20 kick up about 4 inches. We are going to shave the back to get the slope all the way back. After that, I want to run some 4 inch perf tube under the sod. How much gravel should i put arond the tube. Whatever amount it will be wrapped in Geotex , but i just don't know how big the envelope would have to be for it to be effective.